Monodentate pyrazole as a replacement of labile NCS for Ru (II) photosensitizers: Minimum electron injection free energy for dye-sensitized solar cells Hammad Cheema a , Ashraful Islam b , Liyuan Han b , Ahmed El-Shafei a, * a Polymer and Color Chemistry Program, North Carolina State University, Raleigh, NC 27695, USA b Photovoltaic Materials Unit, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan article info Article history: Received 23 January 2015 Received in revised form 31 March 2015 Accepted 1 April 2015 Available online 10 April 2015 Keywords: Dye-sensitized solar cells Carbazole based electron-donor antennas Solar-to-electric conversion Isothiocyanate free Ru (II) bipyridyl complexes Electron injection free energy Impedance spectroscopy abstract A mono-dentate ligand of 3-(triﬂuoromethyl) pyrazole, instead of the labile isothiocyanate ligand for Ru (II) photosensitizers, was developed. This molecular modulation resulted in a new structure motif (HD- 11) with 50 nm red shift in the low energy metal-to-ligand charge transfer (MLCT) absorption peak. However, electrochemical measurements revealed that both ground and excited state oxidation poten- tials were stabilized, we attributed this to the presence of strong electron withdrawing group CF 3 tethered to pyrazole, which resulted in more free energy for dye regeneration (DG regenration ), but less driving force for electron injection (less DG injection ) into the conduction band of TiO 2 . Hence HD-11 , resulted in lower J sc of 12.89 mA cm 2 compared to 17.07 mA cm 2 of N719 under similar conditions. Additionally, V oc of HD-11 was lower than N719, owing to decreased eTiO 2 life time of 0.674 ms compared to 8.8 ms of N719 for injected electrons as found by impedance measurements. © 2015 Elsevier Ltd. All rights reserved. 1. Introduction Since the breakthrough report of adsorbing ruthenium complex on wide band TiO 2 nanocrystalline semiconductor by O 0 Regan and Gr€ atzel [1], huge efforts have resulted in highly efﬁcient DSSC materials [2e7]. Continuous improvement and better under- standing of the device photophysics [8], interface engineering and the inﬂuence of additives, DSSC's with efﬁciency high as 13% (h) [9] has been achieved by Zn-porphyrin complex. Additionally, perovskite-sensitized solid state solar cells can be called as next generation dye-sensitized solar cells have reached conﬁrmed re- cord high efﬁciency of 20.1% [10], whereas efﬁciency high as 19.3% have also been reported [11]. However toxicity related to use of Lead (Pb) and long term stability are still need to be addressed [12]. In the realm of dye sensitized solar cells, photosensitizers are the most crucial functional component at the interface of semi- conductor (TiO 2 ), dye and electrolyte [13,14]. Historically N719 [15,16] (Fig. 1) is the most popular benchmark which served the seminal role in design and engineering of novel photosensitizers to achieve high extinction coefﬁcient [2,17,18], long term stability [19e22], thiocyanate free photosensitizers for high efﬁciency and long term stability [23e28], cyclometalated Ru (II) dyes [29e32] and photosensitizers for NIR response [33e35]. The presence of thiocyanate ligand reduces the yield of the ﬁnal dye due to possible isomerization [36,37]. Also Ru-NCS dative bond has been reported to be prone to decomposition during long term operation of DSSCs [38e41], mechanisms detailing decrease in ef- ﬁciency over long term use of DSSC have been discussed in detail somewhere else [38e41]. Additionally, according to DFT calcula- tions [17,42] sizable contribution of HOMO lies on Ru-NCS bond. Keeping that in mind, replacement of NCS can be judiciously carried out to achieve ﬁne tuning of frontier HOMO and LUMO energy levels, which will eventually affect the dye regeneration and elec- tron injection. Two proven strategies for the replacement of NCS ligand from Ru (II) complexes are by the introduction of pyrazole based anionic ligands of both bi and tridentate nature [23,25,26,28,43,44] and anionic, chelating cyclometalating ligands [29e32]. However monodentate pyrazoles to the best of our knowledge have never been tested for their inﬂuence on frontier orbitals energy and photovoltaic performance as anionic chelates for Ru (II) compounds. * Corresponding author. E-mail address: Ahmed_El-Shafei@ncsu.edu (A. El-Shafei). Contents lists available at ScienceDirect Dyes and Pigments journal homepage: www.elsevier.com/locate/dyepig http://dx.doi.org/10.1016/j.dyepig.2015.04.005 0143-7208/© 2015 Elsevier Ltd. All rights reserved. Dyes and Pigments 120 (2015) 93e98